Communication system and alignment method of transcoder

ABSTRACT

When a terminal is moved to an area covered by a different switching node RNC of a radio access network, degradation of speech quality is prevented by returning a transcoder insertion connection to a transcoder-free-operation connection. When the terminal is moved and the switching node RNC of the radio access network is changed, a relocation is performed by inserting the transcoder, re-confirmation of parameter information of a bandwidth-compression coding system is requested for a local side switching node RNC and a remote side switching node RNC and, when the transcoder can be bypassed according to the confirmation, the transcoder is removed to return to the transcoder-free-operation connection mode.

The present Application is a Continuation Application of U.S. patentapplication Ser. No. 09/935,675, filed on Aug. 24, 2001 now U.S. Pat.No. 7,181,209.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to an interface between a network fortransmitting a bandwidth-compressed signal and a network fortransmitting non-bandwidth-compressed signal. Particularly, in aninterface between a switching node of a radio access network fortransmitting a bandwidth-compressed audio signal and a switching node ofa core network for transmitting a non-bandwidth-compressed audio signalopposing to the radio access network in a communication network to whichthe radio access network and the core network are connected, the presentinvention relates to an insertion/removal control of a transcoderprovided in the switching node of the core network for decoding thebandwidth-compressed audio signal transmitted in the radio accessnetwork. The switching node of the radio access network is referred toas RNC (Radio Network Controller). As the switching node of the corenetwork having a transcoder, a MSC (Mobile Switching Center) having aswitching function and a MGW (Media Gateway Server) having a gatewayswitching function are known. Although the MSC and MGS are differentfrom each other in view of their functions in the core network, the MSCand MGS operate in the same way in view of the insertion/removal controlof the transcoder. Therefore, in the following description, the MSC willbe described as a typical example of the switching node of the corenetwork, which includes the transcoder.

2. Description of the Related Art

As a new mobile communication system, a system, which is constructedwith a core network constructed as a fixed network for performing aposition control, a call control and a service control and a radioaccess network for terminating a radio technology and having aninterface between the switching node RNC of the radio access network andthe switching node MSC of the core network defined as Iu interface, hasbeen proposed. The Iu interface is defined as a hierarchic structureincluding a control plane for exchanging control signals betweenswitching nodes and a user plane for exchanging transmission signals andcontrol signals between users. The user plane is constructed with atransmission portion for transmitting a transmission signal such as anaudio signal and/or data as AAL2 cell in ATM and a control signalportion for performing a negotiation between users. When a communicationis performed between a calling terminal and a called terminal, the audiosignal and/or the data signal is transmitted as a packet signal in ATMafter a negotiation between the terminals is performed through the userplane.

The audio signal is transmitted as a 64 kbps PCM signal within the corenetwork, since, in the core network, there are communications with afixed cable telephone network and communications with other radio accessnetworks. This is because the audio signal is standardized such that itis transmitted as the 64 kbps PCM signal by taking the cable telephonenetwork and the interface with respect to other networks intoconsideration. On the other hand, in order to effectively utilize radiofrequency, which is finite resource, the audio signal to be transmittedis bandwidth-compressed. A codec for bandwidth-compressing the audiosignal is provided in each terminal so that the audio signal istransmitted in the radio access network as a narrow band signal of, forexample, 9.6 kbps. Therefore, a transcoder (TC) for converting abandwidth-compressed audio signal into a 64 kbps PCM signal,transmitting the latter to the side of the core network or converting a64 kbps PCM signal into a bandwidth-compressed audio signal andtransmitting the latter to the radio access network is provided in theswitching node MSC of the core network to perform the conversion betweenthe bandwidth-compressed audio signal and the 64 kbps PCM signal. FIG. 9shows a construction for converting the bandwidth-compressed audiosignal into the 64 kbps PCM signal and transmitting the latter signal.

In FIG. 9, DHT (Diversity Handover Trunk) is provided in the switchingnode RNC on each of a calling side and a called side to play a hand-overfunction in a state of diversity.

Assuming that the called side is a radio terminal utilizing the sameaudio compression coding system, the audio signal is coded/decoded by atleast four coding/decoding devices in the passage from the calling sideto the called side, that is, a codec of the calling terminal, atranscoder of the switching node MSC on the calling side, a transcoderof the switching node of the called terminal and a codec of the calledterminal. Therefore, distortion of the transmission signal due to thecoding/decoding method is accumulated, resulting in a degraded audiosignal quality. If transcoders are inserting into all channels of theswitching node MSC, the cost therefor becomes high and a signal delaymay occur due to the signal conversions.

In a case where one audio compression coding system is used in themobile communication network, the calling and called terminals haveidentical codecs. Therefore, there is no need of converting thebandwidth-compressed signal into the 64 kbps PCM signal by using thetranscoder of the switching node MSC. That is, there is no problem ifthe audio signal is transmitted as a bandwidth compression coded, narrowband signal within the core network without using the transcoder. Insuch case, it is possible to reduce the installation cost of transcodersand, further, the quality of the audio signal can be improved since thenumber of coding/decoding steps to be performed in the communication isreduced. Therefore, in the mobile communication network using one andthe same compression coding system, a system has been employed in whicha bandwidth-compressed audio signal is transmitted as it is by bypassingthe transcoder in the switching node of the core network. This system isreferred to as Transcoder Free Operation (TrFO).

FIG. 10 shows a connection for the Transcoder Free Operation.

In the above mentioned new mobile communication system, it has beenproposed to use an AMR (Adaptive Multi Rate Codec) as the compressioncoding system. The AMR can determine its conversion rate dynamically ina range from 4.75 kbps to 12.2 kbps, for example. Further, the class ofAMR is assigned dependent upon tolerable error rate and the assignmentof the coding frame of the AMR is performed by transmitting a RFCI (Rabsub-Flow Combination Identifier) information assigning a SDU (ServiceData Unit) size of a table describing the conversion type between usersby using the user plane and negotiating mutually between the callingterminal and the called terminal.

As mentioned, in the Transcoder Free Operation using the AMR compressioncoding system, the RFCI information for compression-coding is determinedby users in the user plane of the interface between the switching nodeMSC of the core network and the switching node RNC of the radio accessnetwork. Therefore, when the transcoder is inserted in a communicationpath, the switching node MSC of the core network must insert thetranscoder by executing initializing procedures between the switchingnode MSC and the switching node RNC of the radio access network.

It is assumed that a communication between a calling terminal and acalled terminal is performed by bypassing a transcoder in a switchingnode MSC using the TrFO connection. In such case, one of the terminal,which is moving, enters into an area covered by a switching node RNC ofa different radio access network while switching a base station by thehand-over. This state is shown in FIG. 9. In this case, there may be acase where the terminal moves to a switching node RNC of a differentcommunication carrier. For example, in Europe, there may be acommunication crossing national boundaries. In such case, whenrelocation is performed to a switching node RNC of a different carrier,it may be impossible to continue the TrFO connection since the same RFCIinformation is not always used.

In such case, it is necessary to initialize a local side switching nodeMSC, that is, the relocating terminal side, of a core network byinserting a transcoder, convert an audio signal into a standard 64 kbpsPCM signal, further convert the latter into a bandwidth-compressedsignal corresponding to a remote terminal side and transmit thebandwidth-compressed signal to the remote terminal side. This is becausethere is no guarantee that the same RFCI information for thebandwidth-compression as that previously used can be used for a newswitching node RNC.

As described, when a terminal moves to a switching node RNC of a newradio access network under the TrFO connection, a transcoder is insertedby performing an initialization of transcoder with respect to theswitching node of the new radio access network.

Therefore, when the terminal enters into the area covered by the newswitching node RNC and the transcoder in the switching node MSC isinserted, it is necessary to insert at least four coding/decodingdevices as mentioned previously and degradation of tone quality can notbe avoided.

In order to solve this problem, a method has been proposed (ThroughConnection and Iu User Plane Initialization during TrFO establishment2000.7.16 Siemens 3GPP TSG CN WG4#03 TdocN4-00476). In the proposedmethod, a TrFO connection between a switching node RNC of a radio accessnetwork on a remote side in which a terminal is not moved and aswitching node MSC of a core network is kept continued and a negotiationis performed between the switching node MSC of the core network and theswitching node RNC of the new radio access network on the side of themoving terminal by inserting a transcoder to inquire of the newswitching node about RFCI information. When the RFCI information is thesame as that used in the previous communication and satisfies the TrFOcondition, the system is controlled such that it is shifted to the TrFO.

In this proposal, however, the procedure for shifting the TrFO byinserting/removing the transcoder is complicated, compared with thesystem in which the negotiation is performed after the re-initializationis performed once. That is, when a negotiation is performed between thenew switching node RNC of the radio access network and the switchingnode MSC of the core network and the RFCI information is different fromthat used for the previous TrFO connection, a re-initializationincluding the remote side switching node RNC becomes necessary, so thatthe procedure therefor becomes complicated.

SUMMARY OF THE INVENTION

The present invention was made to solve such problem as mentioned aboveand an object of the present invention is to provide a communicationsystem, which is capable of returning to a TrFO connection call forbypassing a transcoder of a switching node MSC of a core networkimmediately after a terminal is moved to an area covered by a switchingnode RNC of a radio access network, an alignment method of thetranscoder and a switching node.

Another object of the present invention is to provide a communicationsystem in which service degradation, such as delay of connection and/orinstantaneous cut-off of sound, caused by insertion of a transcoder whena terminal is switched to a switching node RNC, an alignment method oftranscoder and a switching node.

Another object of the present invention is to provide a communicationsystem in which a procedure for relocation of a switching node issimple, an alignment method of transcoder and a switching node.

The reason for the insertion of a transcoder in a switching node MSC ofa core network into a TrFO connection call at a time of relocation of aswitching node RNC is that a RFCI information used with respect to anold switching node RNC is not always used for a new switching node RNC.If the RFCI information for the new switching node on an input side ofthe inserted transcoder is the same as that used for the old switchingnode on an output side of the transcoder, AMR data can be transmittedsimply without necessity of a format conversion of AMR data (decoding onthe input side and coding on the output side). Therefore, it is possibleto return to the TrFO connection by bypassing the transcoder.

The present invention is featured by that, when a transcoder of aswitching node MSC of a core network is inserted at a time of relocationof a switching node RNC, the switching node MSC inquires of theswitching node RNC of a radio access network about whether or not asetting information of the inserted transcoder can be initialized byusing a RFCI information, which is the same on both input and outputsides of the transcoder to allow a bypassing of the transcoder, and thetranscoder is removed and the system is returned to a TrFO connectionwhen the switching node RNC is initialized such that the RFCIinformation on the input side of the transcoder becomes the same as thaton the output side of the transcoder. Therefore, it is possible to usethe TrFO connection, which provides a high tone quality, in a newswitching node to which a terminal is moved.

Since the RFCI information assigned by the initialization processing isto assign a format of AMR data toward the new switching node, thepurpose of this system is to assign the RFCI information to the newswitching node RNC by the initialization processing from the switchingnode MSC such that the RFCI information on the input side of thetranscoder becomes the same as that on the output side of thetranscoder.

Incidentally, it is possible that the inquiry from the switching nodeMSC of the core network to the switching node of the radio accessnetwork can be performed by not the user plane of Iu interface but thecontrol plane thereof.

That is, the communication system according to present invention isfeatured by comprising a first network for transmittingnon-band-compressed signal and a second network for transmitting abandwidth-compressed signal, wherein a switching node of the firstnetwork includes a transcoder for performing a conversion between thebandwidth-compressed signal and the non-bandwidth-compressed signal andhas a first mode in which the transcoder is inserted and a signaltransmission is performed by performing the conversion between thebandwidth-compressed signal and the non-bandwidth-compressed signal anda second mode in which the bandwidth-compressed signal is transmitted asit is without the transcoder, and wherein the switching node of thefirst network includes means, which, when a terminal is moved to an areaof a new switching node of the second network during the second mode,inserts the transcoder, inquires of the switching node of the secondnetwork about whether or not a setting information of the transcoder canbe changed to a setting information for the second mode and, when thereis an acknowledge from the switching node of the second network, changesthe mode to the second mode.

Further, the communication system according to present invention isfeatured by comprising a radio access network for transmitting an audiosignal as a bandwidth-compressed signal and a core network fortransmitting an audio signal as a non-bandwidth-compressed signal,wherein a switching node of the core network includes a transcoder forperforming a conversion between the bandwidth-compressed signal and thenon-bandwidth-compressed signal and has a TrFO connection mode in whichthe audio signal is transmitted/received between a calling terminal anda called terminal as the bandwidth-compressed signal without using thetranscoder by a negotiation between the terminals and wherein theswitching node of the core network includes means, which performs acommunication between the calling and called terminals in the TrFOconnection mode and, when one of the calling and called terminals ismoved to an area of a switching node of a new radio access network,inserts the transcoder for the switching node of the new radio accessnetwork, means for inquiring, after the transcoder is inserted, of theswitching node of the new radio access network about whether or not asetting information of the transcoder can be changed to a settinginformation with which a signal format on an input side of thetranscoder becomes the same as that on an output side thereof and meansfor changing the mode between the switching node of the core network andthe switching node of the new radio access network to the TrFOconnection mode when there is an acknowledge from the switching node ofthe new radio access network.

In this communication system, the audio signal bandwidth-compressionsystem in the radio access network may be AMR (Adaptive Multi RateCoding) system, the setting information for bandwidth-compression may beRFCI information indicative of a value of a mapping table fordetermining a conversion rate of an audio signal coding frame in the AMRsystem and the inquiry of the switching node of the radio access networkmay be to inquiring whether or not the setting information is RFCIinformation whose parameter on an input side of the transcoder is equalto a parameter on an output side of the transcoder.

The inquiry of the setting information may use a user plane of Iuinterface defined as an interface between the switching node of the corenetwork and the switching node of the radio access network.

Further, the communication system according to present invention isfeatured by comprising a radio access network for transmitting an audiosignal as a bandwidth-compressed signal and a core network fortransmitting an audio signal as a non-bandwidth-compressed signal, theswitching node of the core network including a transcoder for performinga conversion between the bandwidth-compressed signal and anon-bandwidth-compressed signal and a communication being performed in afirst mode for transmitting a signal by inserting the transcoder andperforming the conversion between the bandwidth-compressed signal andthe non-bandwidth-compressed signal and in a second mode fortransmitting the bandwidth-compressed signal without using thetranscoder, wherein, when, in a communication between a calling terminaland a called terminal in the second mode, one of the calling terminaland the called terminal is moved to an area of a switching node of a newradio access network, the switching node of the core network inserts thetranscoder to change the mode to the first mode, inquires of theswitching node of the new radio access network about whether or not asetting information of the transcoder can be changed to a settinginformation of the transcoder such that the setting information on aninput side of the transcoder becomes the same as that on an output sidethereof and, when the setting information on the input side of thetranscoder is the same as that on the output side thereof, changes themode to the second mode.

The inquiry of the setting information may use a protocol of a userplane of Iu interface defined as an interface between the switching nodeof the core network and the switching node of the radio access networkand the setting information of the transcoder may be a RFCI informationof the AMR coding system.

In a switching node operable in a first mode for performing a conversionbetween a bandwidth-compressed signal and a non-bandwidth-compressedsignal by inserting a transcoder and a second mode for transmitting abandwidth-compressed signal as it is without using the transcoder, thepresent invention is featured by comprising means, which, when aterminal is moved to an area of another switching node in the secondmode, inquires of the switching node about whether or not a settinginformation of the transcoder when the latter is inserted can be changedto a setting information in the second mode and changing it to thesecond mode on the basis of the inquiry.

The switching node is one opposing to the switching node of the radioaccess network for transmitting an audio signal as abandwidth-compressed signal and the setting information may be a RFCIinformation in the AMR system.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments of the present invention will be described with reference tothe accompanying drawings, in which:

FIG. 1 shows a TrFO connection before relocation;

FIG. 2 shows an insertion of a transcoder at a time of relocation;

FIG. 3 shows an alignment operation of a transcoder;

FIG. 4 shows a state when the TrFO connection is restored according tothe present invention;

FIG. 5 is a flowchart showing a transcoder alignment operation accordingto the present invention;

FIG. 6 shows a state where the transcoder alignment is failed;

FIG. 7 shows a state where the transcoder alignment in one direction issuccessful;

FIG. 8 is a flowchart showing the transcoder alignment operation whenrelocation requests occur substantially simultaneously;

FIG. 9 shows a construction in the transcoder connection mode;

FIG. 10 shows a construction of the TrFO connection; and

FIG. 11 shows a system construction of a mobile communication system towhich the present invention is applied.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

A transcoder alignment operation according to the present invention willbe described with reference to FIGS. 1 to 5, in which FIGS. 1 to 4 showsa transcoder alignment operation for returning to a TrFO operation byperforming an inserting/removing operation when a terminal is moved to anew switching node RNC and FIG. 5 is a flowchart of the transcoderalignment operation shown in FIGS. 1 to 4.

In FIG. 1, a TrFO connection is established between a local switchingnode RNC and a remote switching node RNC by bypassing a transcoder of aswitching node MSC with using a RFCI information A of AMR data in an uplink (leftward user bit stream in the drawing sheet) and a RFCIinformation B of AMR data in a down link (rightward user bit stream inthe drawing sheet) according to a negotiation between a calling terminaland a called terminal.

It is assumed here that the calling side terminal is moved to an areacovered by a new switching node RNC of a radio access network as shownin FIG. 2.

In response to a relocation request from an old switching node RNC, theswitching node MSC catches a transcoder and inquires of the oldswitching node RNC about the RFCI information of AMR in the currentlyused the TrFO connection. This is because the RFCI information currentlyused in the TrFO connection is determined by a negotiation between theusers and the switching node MSC does not recognize it. After anacknowledge is acquired from the old switching node RNC, the switchingnode MSC performs a relocation request to a new switching node RNC. Inresponse to the relocation request from the switching node MSC, the newswitching node RNC performs an initialization request of the transcoder.In this step, the transcoder is inserted by initializing the transcoderbetween the switching node MSC and the new switching node RNC, as shownin FIG. 2. Since the initialization request from the new switching nodeRNC is performed by using a RFCI=C (RFCI information of AMR data sent upto the new switching node RNC is C), the switching node MSC inserts thetranscoder by setting the RFCI=C as a setting value of the transcodertoward the new switching node RNC. Since the request from the newswitching node RNC is the RFCI=C, RFCI on an input side of thetranscoder is also set as the RFCI=C. Similarly, the transcoder isinserted for the remote side switching node SNC by using the RFCI=A onthe up link side (input side) thereof and the RFCI=B on the down linkside (output side) thereof on the basis of a response from the oldswitching node RNC.

Incidentally, in FIG. 2, the transcoder of the switching node MSC isshown as a single 2-input, 2-output type transcoder by considering theconstruction of the system in which the coding/decoding is performed onthe input side as well as the output side with respect to the up linkand the down link. Therefore, the transcoder may be constructed with twotranscoders arranged back to back for performing a conversion of AMRdata into a 64 kbps PCM signal and a conversion of a 64 kbps PCM signalinto AMR data.

Considering the RFCI information on the input side of the transcoder andthe RFCI information on the output side of the transcoder, thetranscoder is not requested to perform a format conversion(coding/decoding processing) of AMR if the RFCI information on the inputside thereof is the same as the RFCI information on the output sidethereof. That is, it is enough for the switching node MSC to merely passand transmit only AMR data (bandwidth-compressed audio signal), so thatit is possible to avoid degradation of tone quality by merely removingthe transcoder. If the RFCI information is common for the input side andthe output side in both the up and down directions, the transcoderitself becomes unnecessary. Therefore, it is possible to return to theTrFO connection call by removing the transcoder.

Therefore, by considering the RFCI information on the output side of thetranscoder, the switching node MSC requests the switching nodes RNC onthe input side of the transcoder to initialize with using the outputside RFCI information. In FIG. 3, the switching node MSC requests thenew switching node RNC on the local side to initialize with using RFCI=Bof AMR data outputted to the remote side switching node RNC. Further,the switching node MSC requests the remote side switching node RNC toinitialize with using RFCI=C of AMR data outputted to the local sideswitching node RNC. When acknowledges for the initialization requestsare received from the respective switching nodes RNC, the up directionbecomes RFCI=C and the down direction becomes RFCI=B and the transcodercan be bypassed in both directions. Therefore, as shown in FIG. 4, theswitching node MSC is returned to the TrFO connection for AMR datahaving RFCI=C in the up direction and RFCI=B in the down direction byremoving the transcoder thereof.

FIGS. 6 and 7 shows the functions of the transcoder by symbolizing thestates of connection thereof. The transcoder has the function ofoutputting AMR data from the local switching node RNC to the remoteswitching node RNC by converting the AMR data and the function ofoutputting AMR data from the remote switching node RNC to the localswitching node RNC by converting the AMR data. These functions can berepresented by symbolizing them as data format conversion functions.FIG. 6 shows an example in which the transcoder alignment is failed andthe returning to the TrFO connection call is failed and FIG. 7 shows anexample in which the transcoder alignment in the up direction issuccessful and the transcoder in the up direction is removed. Even inthe case in which the transcoder in one direction is removed, it ispossible to reduce the number of transcoders inserted into thetransmission path to thereby improve the tone quality.

In a case where the called side terminal is moved and the relocation ofthe switching node RNC is performed, the above mentioned processing isperformed in the remote side switching node MSC.

In a case where the calling side and the called side perform therelocation request of the respective switching nodes RNC, the callingside switching node MSC and the called side switching node MSC performthe processing shown in FIG. 1 to FIG. 5 within a time for which soundis discontinued, respectively. Therefore, there is no problem unless thetransmitting side and the receiving side perform the relocation requestssubstantially simultaneously.

FIG. 8 shows a flowchart when the transmitting side and the receivingside request relocations substantially simultaneously. In this case, itis assumed that RFCI information used in the TrFO connection is RFCI=Ain the up direction and RFCI=B in the down direction and theinitialization requests from a new switching node RNC (L-RNC) on thetransmitting side (local side) and a new switching node RNC (R-RNC) onthe receiving side (remote side) are RFCI=C and RFCI=D, respectively.The local side switching node L-MSC and the remote side switching nodeR-MSC inquire of the old switching nodes L-RNC and R-RNC about RFCIinformation in use and set the transcoders on the basis of theacknowledges therefor and the initialization requests from the newswitching nodes L-RNC and R-RNC. Then, the alignment processing isperformed for the thus set transcoders.

In this case, the output of the switching node L-MSC to the core networkside is RFCI=B. Therefore, when the switching node L-MSC inquires of thenew switching node L-RNC on the transmitting side about theinitialization request with using this RFCI information, the request isacknowledged. However, the initialization request to the remote sidewith using RFCI=C is rejected since the initialization request from thenew switching node R-RNC on the receiving side is RFCI=D. Therefore,data in the down direction (rightward direction) becomes RFCI=B in theswitching node L-MSC and the transcoder can be removed for the data inthe up direction, so that it is possible to return to the TrFOconnection. As to the up direction (leftward direction), data of RFCI=Ais converted into AMR data of RFCI=C by the transcoder and transmitted.Similarly, in the switching node R-MSC on the receiving side, theinitialization request for making the up side data for the new switchingnode R-RNC RFCI=A is acknowledged and the initialization request formaking the down side data for the local side RFCI=D is rejected.Therefore, the RFCI information on the input side of the transcoder isthe same as that on the output side thereof in the up direction, so thatthe transcoder is removed. For the down direction, data of RFCI=B isconverted into data of RFCI=D by the transcoder and transmitted.

This case is the same as the case shown in FIG. 7 in which the alignmentin one direction is failed and the transcoder is inserted in onedirection and one transcoder is removed in the local side switching nodeMSC and one transcoder is removed in the remote side switching node MSC,so that the number of transcoders inserted into the transmission pathcan be reduced.

As described hereinbefore, according to the present invention, tonequality in one or both directions, which is degraded by insertion of atranscoder in the relocation of a switching node of a radio accessnetwork, can be improved by removing the transcoder by performed thealignment processing of the setting information of the transcoder.Further, since the TrFO connection becomes possible by the alignmentprocessing of the setting information of the transcoder, it is possibleto reduce the number of transcoders provided in the switching node ofthe core network to thereby reduce the cost of the communication system.

1. A method used in a communications system including a switching node,a first radio access switching node, and a second radio access switchingnode, said method comprising: inserting a transcoder in said switchingnode before performing a relocation from said first radio accessswitching node to said second radio access switching node; initializingsaid second radio access switching node so that compression codinginformation from said second radio access switching node becomes thesame as compression coding information from said first radio accessswitching node if the compression coding information is not the same;and removing said transcoder after making the compression codinginformation the same.
 2. The method according to claim 1, wherein saidswitching node is in a core network.
 3. The method according to claim 2,wherein said switching node comprises a mobile switching center (MSC).4. The method according to claim 2, wherein said switching nodecomprises a media gateway server (MGW).
 5. The method according to claim1, wherein said first and second switching nodes each comprises a radionetwork controller (RNC).
 6. The method according to claim 1, whereinsaid transcoder is applicable to adaptive multi-rate (AMR) codec.
 7. Themethod according to claim 1, wherein said compression coding informationincludes RFCI (Rab sub-Flow Combination Indicator) information.
 8. Themethod according to claim 1, wherein said mobile terminal communicatesunder a transcoder-free operation (TrFO) after a removal of saidtranscoder.
 9. The method of claim 1, wherein said communication systemperforms a transcoder free operation (TrFO).
 10. A method used in aswitching node, comprising: inserting a transcoder in said switchingnode before performing a relocation from a first radio access switchingnode to a second radio access switching node; initializing said secondradio access switching node so that compression coding information fromsaid second radio access switching node becomes the same as compressioncoding information from said first radio access switching node if thecompression coding information is not the same; and removing saidtranscoder after making the compression coding information the same. 11.A communications system, comprising: a switching node; a first radioaccess switching node that covers a first area; and a second radioaccess switching node that covers a second area, wherein said switchingnode comprises a transcoder, wherein said transcoder is inserted in saidswitching node before performing a relocation from said first radioaccess switching node to said second radio access switching node, andwherein said switching node initializes said second radio accessswitching node so that compression coding information of said secondradio access switching node becomes the same as compression codinginformation from said first radio access switching node if thecompression coding information is not the same and removes saidtranscoder after making the compression coding information the same. 12.The communications system according to claim 11, wherein said switchingnode is in a core network.
 13. The communications system according toclaim 12, wherein said switching node comprises a mobile switchingcenter (MSC).
 14. The communications system according to claim 12,wherein said switching node comprises a media gateway server (MGW). 15.The communications system according to claim 11, wherein said first andsecond radio access switching nodes each comprises a radio networkcontroller (RNC).
 16. The communications system according to claim 11,wherein said transcoder is applicable to adaptive multi-rate (AMR)codec.
 17. The communications system according to claim 11, wherein saidcompression coding information comprises RFCI (Rab sub-Flow CombinationIndicator) information.
 18. The communications system according to claim11, wherein said mobile terminal communicates under a transcoder-freeoperation (TrFO) after a removal of said transcoder.
 19. Acommunications system, comprising: switching means for carrying out aswitching or gateway switching function; first radio access switchingmeans for switching for a first area; and second radio access switchingmeans for switching for a second area, wherein said switching meanscomprises transcoding means for transcoding a signal, wherein saidtranscoding means is inserted in said switching means before performinga relocation from said first radio access switching means to said secondradio access switching means, and wherein said switching meansinitializes said second radio access switching means so that compressioncoding information from said second radio access switching means becomesthe same as compression coding information from said first radio accessswitching means if the compression coding information is not the same,and removes said transcoding after making the compression codinginformation the same.
 20. A switching node, comprising: a transcoder,wherein said transcoder is inserted in a switching node beforeperforming a relocation from a first radio access switching node to asecond radio access switching node, and wherein the switching nodeinitializes said second radio access switching node so that compressioncoding information from the second radio access switching node becomesthe same as compression coding information from said first radio accessswitching node if the compression coding information is not the same andremoves said transcoder after making the compression coding informationthe same.
 21. A switching node, comprising: switching means for carryingout a switching or gateway switching function; and transcoding means fortranscoding a signal, wherein said transcoding means is inserted in theswitching node before performing a relocation from a first radio accessswitching node to a second radio access switching node, and wherein theswitching node initializes said second radio access switching node sothat compression coding information from said second radio accessswitching node becomes the same as compression coding information fromsaid first radio access switching node if the compression codinginformation is not the same and removes said transcoding means aftermaking the compression coding information the same.